Field of the Invention
The present invention is related to a method for comparing frequency differences between optical signals, a method for synchronizing phases of optical signals and a device for synchronizing phases of optical signals.
Description of the Related Art
It is possible to transmit highly stable high-frequency reference signals for long distances by using two coherent laser waves. This allows construction of extensive coherent systems. A high-frequency reference signal, transmitted as a beat signal between two light waves, is subjected to optical-electric signal conversion, and then phase-synchronized with an electric oscillator by using a phase synchronization circuit so that a new frequency is generated. Today, reference signals transmitted by two light waves span a wide frequency band ranging from low frequencies to a frequency of more than 100 GHz. In order to do this, various types of photomixers or frequency converters used for optical-electric signal conversion have to be provided ranging from those for low frequencies to those for high frequencies. A good example of this is ALMA (Atacama Large Millimeter/submillimeter Array). The construction of ALMA is described in J. F. Cliche, et al, “A 100-GHz-tunable photonic millimeter wave synthesizer for the Atacama Large Millimeter Array radio telescope”, IEEE/MTT-S International Microwave Symposium, 2007, p. 349-352.
Great developments are expected in the future in the field of generating highly phase-stable signals from the microwave region to the terahertz wave region. The need thereof is growing in high speed optical communication, high-frequency astronomy, etc.
Using photonic technologies is advantageous for generating high-frequency wideband signals. Also, optical signals can be easily distributed for long distances by using fiber transmission. Recently, systems constituting an extensive coherent system by distributing a high-frequency optical reference signal in a broad area are beginning to be realized. A reference signal in a high frequency is advantageous for maintaining coherence because the signal does not require any multiplier or the like at the distributed destination. A good example of this is the ALMA. A high-frequency optical reference signal is generated and transmitted as a beat signal between two coherent laser waves. In conventional techniques, this high-frequency optical reference signal is converted into an electric signal by optical-electric signal conversion using a photomixer or the like to provide a frequency reference, which is used for comparison with a controlled signal as to their frequency differences.
FIG. 5 shows an example of a conventional frequency comparison method. This construction corresponds to a scheme referred to as PLL (Phase Locked Loop). A “two light wave generation means” generates an optical signal including two light waves. The optical signal is distributed in two paths by an “optical distributor”. One of the distributed optical signals is subjected to optical-electric conversion and an output signal is outputted as a high-frequency electric signal having a frequency corresponding to a frequency difference between the two light waves. The other signal is also subjected to optical-electric conversion and a phase difference between this and a reference optical signal, which has been subjected to optical-electric conversion in a similar manner, is detected. The frequency difference of the two light waves (the optical signal) generated by the “two light wave generation means” is controlled based on this phase difference.
FIG. 6 shows another example of a conventional frequency comparison method. This construction represents a PLL oscillator synchronized with an optical reference signal. This example uses a “high-frequency electric signal generation means” instead of the “two light wave generation means”. The signal from the “high-frequency electric signal generation means” is distributed in two paths, one of which is an output signal. A phase difference between the other path and an optical reference signal, subjected to optical-electric conversion, is detected. The frequency of the electric signal generated by the “high-frequency electric signal generation means” is controlled based on this phase difference.
Thus, in both the conventional examples, an optical-electric converter is required for detecting the frequency difference between two light waves at least in the optical reference signal. This optical-electric converter has to be operational over an entire range of possible frequencies for the optical reference signal.